Intensity of perioperative analgesia but not pre‐treatment pain is predictive of survival in dogs undergoing amputation plus chemotherapy for extremity osteosarcoma

Abstract The purpose of this bi‐institutional retrospective study was to determine whether, in dogs treated with limb amputation and adjunctive chemotherapy for osteosarcoma, oncologic outcomes are impacted by either: (1) baseline cancer pain severity, or (2) the approaches used for perioperative pain management. Data were extracted from the medical records of 284 dogs that underwent both limb amputation and chemotherapy (carboplatin and/or doxorubicin) between 1997 and 2017 for localized (non‐metastatic) osteosarcoma of the appendicular skeleton. Kaplan–Meier survival curves and Cox proportional hazard (PH) models were used to determine the impact that retrospectively scored baseline pain levels (high vs. low) and various analgesic and local anaesthetic treatments had on both metastasis‐free survival and all‐cause mortality. For the entire population, the median disease free interval and median overall survival times were 253 and 284 days, respectively. Baseline pain was rated as “low” in 84 dogs, and “high” in 190 dogs; pain severity had no detectable effect on either metastasis‐free survival or all‐cause mortality. When accounting for the potential influences of known prognostic factors, dogs treated with what was characterized as a high‐intensity perioperative analgesic plan (including both a non‐steroidal anti‐inflammatory drug [NSAID] and a bupivacaine‐eluting soaker catheter placed at the amputation site) had a higher probability of survival than dogs treated with a low‐intensity perioperative analgesic plan (neither an NSAID, nor a soaker catheter); the median overall survival times were 252 and 378 days, respectively (hazard ratio: 2.922; p = .020).

on quality of life. [1][2][3][4] Osteosarcoma-associated pain may also influence prognosis. A recent retrospective study provides evidence that in dogs undergoing chemoradiotherapy for extremity osteosarcoma, high pain is predictive of short overall survival. 5 It is unclear whether pain is simply a bellwether for early osteosarcoma-associated death, or if there is a true mechanistic link between pain and cancer progression. One consideration is that the pro-nociceptive ligands being produced during tumour progression and leading to generation of pain signals (e.g., nerve growth factor, endothelin-1, prostaglandin E2, etc.) may also be promoting aggressive tumour behaviours. 6,7 Prognosis may also be influenced by both the pain caused by cancer treatment, and the manner in which that pain is managed. Experimentally, surgical pain can enhance retention of cancer cells in the lungs of rats. 8 That effect can be mitigated with morphine. 9 However, there is also concern that opioids themselves may hasten cancer recurrence or metastasis, either as a direct effect of the drugs, or perhaps opioid-induced immunosuppression enables escape of cancer cells from immune surveillance. 10,11 Similar observations have been made in humans. It has been reported that after transurethral resection of superficial bladder cancer, recurrence rates are lower when spinal anaesthesia is used instead of general anaesthesia. 12 Similarly, following open thoracotomy for resection of primary lung tumours, overall survival was longer in patients treated with a paravertebral block, versus intravenous patient-controlled analgesia. 13 These retrospective studies are limited by small sample size and failure to account for all potential confounders and known prognostic factors; there are also similarly designed retrospective studies, and even some well-designed prospective investigations that fail to find an association between the analgesic/anaesthetic approach and oncologic outcomes. [14][15][16] Thus, the potential for pain and pain management to influence oncologic outcomes may be restricted to certain malignancies, patient populations, and treatment approaches.
In dogs with non-metastatic osteosarcoma, the most widely accepted definitive-intent treatment is limb amputation followed by adjunctive chemotherapy (most often carboplatin and/or doxorubicin).
In dogs treated as such, it remains uncertain whether baseline tumour-associated pain severity is predictive of long-term survival. It is also unknown whether surgical pain, or the choice of which perioperative analgesics are used to manage that pain, might influence outcomes. Therefore, the purpose of this study was to determine whether progression-free or overall survival times are associated with either: (1) retrospectively assigned baseline pain severity scores, or (2) the types of analgesics and local anaesthetics that were used before, during, and in the first few days following surgical limb amputation.

| Data acquisition
A bi-institutional retrospective study was performed. Institutional animal care and use committee approval was not required. For each included case, standard written pet owner consent for treatment was obtained; case management was at the discretion of the medical team managing the case at the time. Oncology accession logs (February 1997-September 2017) at two academic veterinary teaching hospitals were searched for cases in which limb amputation and at least one dose of adjuvant chemotherapy (carboplatin, doxorubicin, or a combination thereof) was used as treatment for histologically confirmed osteosarcoma in dogs that were free of both lymph node metastasis and pulmonary metastasis (as determined by three-view thoracic radiographs). Dogs were excluded if they had pathologic evidence of lymph node metastasis; in cases for which cytology and/or histopathology was not performed, nodes were deemed "normal" whenever there was absence of palpable regional lymphadenopathy in recorded physical exam data. Cases were excluded if body weight at diagnosis was less than 15 kg. 17 Cases were also excluded if there was lack of access to the histopathology report, inability to ascertain the nature of chemotherapy protocols, or insufficient follow-up to ascertain overall survival time.
The following data were extracted from the medical records: breed, sex and neuter status, date of birth, body weight at diagnosis, tumour location, preoperative total serum alkaline phosphatase (ALP) concentration, preoperative absolute monocyte count, date of amputation, types of analgesics and local anaesthetic techniques used perioperatively, details of chemotherapy, date and site of metastasis, date of last follow-up, and date and cause of death. A baseline pain score was retrospectively assigned as either "high" or "low", as previously described. 5 Classification of pain as "high" required documentation of a pain score of 3 or 4 out of 4, presence of a non-weight bearing lameness, and/or description of moderate-to-severe pain in the physical examination; "low" pain was defined by a pain score of 0, 1, or 2 out of 4, weight-bearing lameness, and/or description of absent to mild pain on physical examination. 18 Pain scores, based on the Colorado State University Acute Pain Scale, were not assigned retrospectively; we only used contemporaneous observations that were made by the attending clinician and recorded in the medical record at the time of assessment. No attempt was made to record use of nonpharmacologic analgesia strategies (e.g., acupuncture). Data were not collected regarding: histopathologic margin status, chemotherapy dosing or dose interval, or adverse effects of treatment.

| Statistical analyses
Overall survival time (OST) was defined as the time from amputation to death. If lost to follow-up, cases were censored at the time of last contact, and cases were also censored if alive at the time of analysis.
Progression-free survival (PFS) was defined as the time from amputation until identification of (radiographically or pathologically diagnosed) metastatic disease; subjects were censored if alive without evidence of tumour progression at the time of analysis, or if the cause of death was demonstrably unrelated to OS. Kaplan-Meier methods were used to calculate median PFS and OST with 95% confidence intervals (CI). Factors evaluated for potential predictive or prognostic value included: age; sex; body weight; tumour location      In univariate testing (results of which are summarized in Table 1

| DISCUSSION
In this study, baseline (preamputation) osteosarcoma-associated pain was not associated with PFS or OST. While the intensity of overall analgesic support was not associated with PFS, overall survival was prolonged in dogs receiving what was defined as a high-intensity multimodal perioperative analgesic protocol.
A previous study reported that, in dogs treated with combinatorial chemoradiotherapy, high baseline osteosarcoma-associated pain was associated with short overall survival. 5 That observation was made in a relatively small sample size wherein pain severity was retrospectively classified. To build upon that observation and gain initial insight as to whether high pain may indicate a biologically aggressive tumour phenotype that is likely to metastasize sooner than a tumour with low pain, we used similar methods here to retrospectively assess this population of 284 dogs. Acknowledging the same methodologic limitations (namely that the Colorado State University acute pain scoring system has not been rigorously validated as a useful readout of pain in this disease and treatment setting, and furthermore, there has also been no validation of the methods for retrospectively bucketing baseline pain as "high" or "low"), the data and analyses presented herein do not suggest that severity of baseline tumour-associated pain is predictive of either metastasis-free or overall survival in dogs undergoing limb amputation plus adjunctive chemotherapy for extremity osteosarcoma. Addressing the above experimental design limitations will be required before firm conclusions can be drawn, and ideally that validation will take the form of a prospective study in which rigorous methods are used for pain assessment.
In addition to pain caused by the primary tumour, amputation is associated with pain, and later (chronic/persisting) post-amputation neuropathic-like pain has recently been reported to affect about onethird of dogs. 20 Dogs in the current study had no consistent, standardized postoperative pain assessment data available for review. Instead, we indirectly assessed postoperative pain by looking at how postoperative pain was clinically managed. This assessment of analgesic and local anaesthetic usage is limited by the fact that any measured effect may not relate to pain per se, but rather, to direct effects of analgesics. It is also limited by the fact that we only know what was given in-hospital and what was prescribed for post-discharge outpatient use, but we did not have any way to retrospectively determine pet owner compliance with post-discharge instruction; data around use of NSAIDs in the home environment must be considered on an "intent In line with the exploratory nature of this work, our next step was to assess other patterns of analgesic use in the perioperative period.
We found no association between progression-free or overall survival, and either: (a) preoperative use of NSAIDs or non-NSAIDs, analgesia. [42][43][44] We found that in this population of dogs, highintensity analgesic support (as defined in our approach) was strongly and significantly associated with prolonged overall survival. Given that all dogs had undergone limb amputation, this suggests a difference in time to metastasis. That interpretation is limited by the fact that we evaluated all-cause survival, rather than disease-specific survival. We did not find a difference between the groups in terms of progressionfree (i.e., metastasis-free) survival; however, the PFS analysis itself was limited by low quality data, which resulted from the fact that standardized restaging protocols were not in place for the cases included in this study. We did find a positive effect on overall survival from high-intensity analgesic support and there are several possible explanations for this intriguing observation. First, it is possible that the high-intensity multimodal analgesic support package may have reduced clinician reliance upon opioids (which have been shown experimentally to dampen anti-tumour immunity and promote aggressive tumour behaviours such as metastasis); however, this is not supported by our data, which indicate that care for dogs in the highintensity group was also characterized by more frequent use of morphine-containing epidurals, longer use of opioids delivered via continuous rate infusion postoperatively, and more frequent postoperative use of transdermal fentanyl patches. 8,45 Second, provision of effective analgesic therapy may alter (reduce) the accumulation of pro-algesic factors (e.g., neurotrophins) in local tissues and systemic circulation; this is potentially significant because many factors which modulate pain are also known to modulate tumour behaviour. 6,7 Third, the analgesic drugs used may have had direct, combined anticancer effects on micrometastases present at the time of analgesic provision, as both NSAIDs and local anaesthetics have been shown to have anti-cancer properties. 31,46,47 Fourth, presence of pain may alter the psychological state of these animals, which can in turn alter immune function in such a way that is permissive of tumour cell growth and/or metastasis. 48 These potential explanations are not mutually exclusive. Finally, the result could be a false positive; perhaps there is some other unmeasured or un-appreciated factor (e.g., selection bias) that was in common for the subset of 16 dogs that had high-intensity analgesic support.
An important potential limitation of this work is that the data were